The continuous or partially discontinuous preparation of MDA is disclosed in, for example, EP 1 616 890 A1, U.S. Pat. No. 5,286,760, EP-A-451442 and WO-A-99/40059. The acidic concentration of aromatic amines and formaldehyde to form diamines and polyamines of the diphenylmethane series proceeds in a plurality of reaction steps.
In the aminal process, formaldehyde is firstly condensed with aniline in the absence of an acid catalyst to form aminal, with water being eliminated. The rearrangement to form MDA is then carried out in the presence of an acid catalyst in a first step to form para- and ortho-aminobenzylaniline. The aminobenzylanilines rearrange in a second step to form MDA. Main products of the acid-catalyzed reaction of aniline and formaldehyde are the diamine 4,4′-MDA, its positional isomers 2,4′-MDA and 2,2′-MDA and also higher homologs.
In the neutralization process, aniline and formaldehyde are converted directly in the presence of an acid catalyst into aminobenzylanilines which subsequently react further to form the two-ring MDA isomers and MDA homologs having more than two rings.
Regardless of the process variant for preparing the acidic reaction mixture, the work-up of this is, according to the prior art, commenced by neutralization by means of a base. The neutralization is usually carried out at temperatures of, for example, from 90° C. to 100° C. without addition of further substances. (H. J. Twitchett, Chem. Soc. Rev. 3(2), 223 (1974)). However, it can also be carried out at a different temperature level, for example in order to accelerate the degradation of interfering by-products. Hydroxides of the alkali and alkaline earth elements are suitable as bases. Preference is given to using aqueous NaOH.
After neutralization, the organic phase is separated from the aqueous phase in a separation vessel. The organic phase containing crude MDA which remains after the aqueous phase has been separated off is subjected to further work-up steps, for example, a wash with water (base wash) in order to wash residual salts out from the crude MDA. The crude MDA which has been purified in this way is finally freed of excess aniline, water, and other materials (e.g. further solvents) present in the mixture by suitable processes such as distillation, extraction or crystallization. The work-up which is routine according to the prior art is disclosed, for example, in EP 1 652 835 A1, page 3, line 58 to page 4, line 13, or EP 2 103 595 A1, page 7, lines 21 to 37.
EP 2 486 975 A1 discloses the use of a specific mixer-reactor in the preparation of MDA. It is stated that a local excess of formaldehyde can lead to the formation of network-like polymers. However, the patent application gives no details about the configuration of the reaction running-down procedure, i.e. interruption of the process. In particular, the patent application does not teach that the “A/F ratio” (the molar ratio of aniline to formaldehyde) during the running-down procedure should be above the A/F ratio during normal operation.
EP 1 616 890 A1 teaches that aniline and formaldehyde are firstly reacted in the absence of the acid catalyst to form aminal and the aminal is subsequently admixed with an acid catalyst and is reacted further at temperatures of from 20° C. to 100° C. and at water contents of the acidic reaction mixture obtained in this way of from 0 to 20% by weight. In particular, the water is firstly at least partly removed from the aminal after condensation of formaldehyde and aniline to form aminal, with a water content of from 0 to 5% by weight in the aminal being set, and the aminal is subsequently admixed with an acid catalyst and is reacted further at temperatures of from 20° C. to 100° C. and water contents of the acidic reaction mixture obtained in this way of from 0 to 20% by weight. Mixtures of diamines and polyamines of the diphenylmethane series having degrees of protonation of <15%, preferably from 4% to 14%, particularly preferably from 5% to 13%, can be prepared in this way. Here, the degree of protonation is, in the case of monoprotic acid catalysts (e.g. hydrochloric acid), the molar ratio of the amount of acid catalyst used and the molar amount of amine functions present in the reaction mixture. The patent application does not given any details regarding the procedure during running-down of an industrial production plant. The examples gives are laboratory experiments. In particular, this patent application does not teach that the A/F ratio during the running-down procedure should be above the A/F ratio during normal operation.
EP 0 283 757 A1 is likewise concerned with the preparation of MDA. The process described is characterized by the addition of aniline-free MDAs to aminobenzylamines formed by condensation of aniline and formaldehyde before the rearrangement reaction of these induced by heat. Example 2 describes a mode of operation in which a small part of the MDA formed is recirculated to the rearrangement reaction (cf. also claim 8). In other words: the configuration of an MDA plant in continuous normal operation is described. Details of the procedure when running-down an MDA plant are not described; in particular, there is no information about the A/F ratio during running-down compared to the A/F ratio during the reaction.
WO-A-99/40059 teaches that, in order to prepare methylenedi(phenylamine) by reaction of aniline with formaldehyde in the presence of acid catalysts in a semicontinous process, aniline and optionally acid catalyst are initially charged, formaldehyde and optionally acid catalyst are fed through a mixing device into a circuit in which aniline, optionally acid catalyst and optionally previously introduced formaldehyde are circulated, and the reaction mixture is brought to a temperature of greater than 75° C. after introduction of at least 50% of the total amount of formaldehyde to be fed in. The addition up to an amount of at least 50% of the total amount of formaldehyde to be fed in is carried out at a temperature of the reaction mixture in the circuit of from 20° C. to 75° C.
None of the above-mentioned document of the prior art suggests using a A/F ratio which differs from that during normal operation during running-down of the reaction for preparing MDA. It is thus quite routine in the prior art to use A/F ratios during normal operation which are above that conforming to the stoichiometry of the reaction (2:1). However, the prior art does not in any way suggest maintaining even greater A/F ratios during running-down.
The quality of a process for preparing MDA is defined firstly by the content of undesirable by-products of the reaction in the product. Secondly, the quality of a process is defined by the overall process from start-up, normal production to running-down of the process being able to be operated without technical failure of production or problems which require intervention in the process and by there being no losses of starting materials, intermediates or end product.
Such problems can, for example occur during taking the aminal reaction out of operation (during “running-down”). Such problems can, for example, be that there is formation of high molecular weight solids which lead to caking and blockages on the equipment (aminal vessel, aminal cooler and aminal separator and conduits).
Although the above-described processes of the prior art are able to produce MDA in high yield without a reduction in quality of the end products, only processes which are in normal operation are described. Running-down processes until stoppage of the plant is attained (known as “running-down time”) are not taken into account.
Start-up and running-down times frequently occur in everyday industrial operation and are not necessarily associated with opening or other mechanical intervention in a reactor or other apparatus of the plant, but can also be associated with shutting-down and restarting of the production plant for various other reasons, e.g. lack of raw materials. These running-down times are in practice characterized in that deviations in the desired molar ratio of aniline to formalin can occur.